1. Field of the Invention
This invention relates to an aircraft starter and more particularly to a control system for a starter valve which regulates air flow to an air turbine starter for an aircraft.
2. Description of Prior Art
Air turbine type starter motors, such as the Bendix 36E163 Air Turbine Starter, operating with the energy of a compressed gas such as air, are often used for starting an aircraft gas turbine engine. The compressed air for the air turbine starter is controlled by a starter valve, such as a Bendix 38E73 air regulating and shut-off butterfly valve.
A source of relatively clean dry air is required to power the air turbine starter. The most common source of air for this purpose are an accessory power unit, bleed air from the compressor stage of another operating gas turbine engine, or a gas turbine ground cart. Upon actuation of the engine start switch, the starter valve is energized permitting air to flow to the air turbine starter. The air turbine starter output engages the engine which is then accelerated to a predetermined cut off speed. The start cycle may be terminated manually by the pilot opening the start switch or automatically by a speed sensitive switch built into the starter. When the start cycle is terminated, the starter valve is closed cutting off the compressed air supply to the air turbine starter. When starting air is cut off, the air turbine starter will automatically disengage from the engine accessory drive shaft and come to a stop.
The starter valve controls the operation of the air turbine starter by means of a controlled opening rate of the valve, a controlled closing rate, and/or a pressure regulating system which delivers substantially constant pressure to the starter regardless of the upstream air pressure. These functions in a conventional starter control valve are implemented by mechanical-pneumatic control devices such as orifices, needle valves, springs and diaphrams. Limitations of these devices include design and manufacturing complexity, difficulty of adjustment, sensitivity to environmental changes and poor repeatability.
An aircraft starter control system for automatically closing a solenoid operated air valve, that supplies pressurized air to rotate an air turbine starter engine of an aircraft engine, when the starter rotor reaches a predetermined rotational speed, is described in U.S. Pat. No. 3,812,378 by inventor William E. Coman. A permanent magnet attached to the air turbine starter rotor induces pulses in a coil which through a control circuit causes shut off of the pressurized air supply when the air turbine starter reaches a predetermined speed.